Conventional graphics processing systems sometimes implement techniques for clipping graphics primitives. Clipping typically refers to a set of operations that determine which portion of a graphics primitive is to be displayed with respect to a set of clipping planes. Various techniques have been developed for clipping graphics primitives. Examples of these techniques include the Cohen-Sutherland technique, the Liang-Barsky technique, the Sutherland-Hodgeman technique, and the Weiler technique. Of these techniques, the Sutherland-Hodgeman technique is perhaps the most commonly used.
FIG. 1 illustrates an example of a graphics primitive 100 that is clipped in accordance with a conventional clipping technique, such as the Sutherland-Hodgeman technique. As illustrated in FIG. 1, the graphics primitive 100 is defined by a set of vertices (V0, V1, V2), such that different pairs of the vertices define various edges of the graphics primitive 100. For example, the pair of vertices (V0, V1) define an edge 102 of the graphics primitive 100. In the illustrated example, each vertex Vi is represented by a set of coordinates (xi, yi, zi, wi) in homogeneous space. For example, the vertex V0 is represented by a set of coordinates (xo, yo, zo, wo), and the vertex V1 is represented by a set of coordinates (xl, yl, zl, wl).
Referring to FIG. 1, the graphics primitive 100 is clipped with respect to a particular clipping plane 104 that defines a viewing region. The clipping plane 104 can be, for example, a left plane (i.e., x=−w plane), a right plane (i.e., x=+w plane), a bottom plane (i.e., y=−w plane), a top plane (i.e., y=+w plane), a near plane (i.e., z=−w plane), or a far plane (i.e., z=+w plane). To clip the graphics primitive 100 with respect to the clipping plane 104, the conventional clipping technique determines a clipping distance di of each vertex Vi with respect to the clipping plane 104. If the vertex Vi lies inside of the clipping plane 104, the clipping distance di is typically assigned a positive value. On the other hand, if the vertex Vi lies outside of the clipping plane 104, the clipping distance di is typically assigned a negative value. For example, to clip the edge 102 with respect to the clipping plane 104, the conventional clipping technique determines a clipping distance d0 of the vertex V0 as well as a clipping distance d1 of the vertex V1. Once the clipping distances d0 and d1 are determined, the conventional clipping technique determines a new vertex Va, which corresponds to an intersection of the edge 102 with respect to the clipping plane 104. In particular, the new vertex Va is represented by a set of coordinates (xa, ya, za, wa), which are determined based on the clipping distances d0 and d1, the set of coordinates (xo, yo, zo, wo), and the set of coordinates (xl, yl, zl, wl).
Conventional clipping techniques are commonly implemented using micro-code, such that a clipping program specifies a sequence of clipping operations to be performed. In accordance with such implementation, a processing time to clip a graphics primitive is typically dependent upon a complexity and a size of the clipping program. Unfortunately, a conventional clipping technique typically involves performing different types of operations for different clipping planes, and implementing these operations using micro-code often results in a clipping program that is undesirably complex and long.
For example, in the case of the Sutherland-Hodgeman technique, operations to determine a clipping distance di of a vertex Vi is typically dependent upon which clipping plane is involved. The following represents a portion of a conventional clipping program to determine the clipping distance di:                (1) If left plane                    then di=wi−xi                          Else branch        (2) If right plane                    then di=wi−xi                          Else branch        (3) If bottom plane                    then di=wi−yi                          Else branch        (4) If top plane                    then di=wi+yi                          Else branch        (5) If near plane                    then di=wi−zi                          Else branch        (6) If far plane                    then di=wi+zi As can be appreciated, the clipping program includes separate sets of instructions and multiple branches to account for different clipping planes. As a result, the clipping program can be slow to execute, thus translating into a processing time that is longer than desired. Also, the processing time can be relatively unpredictable, particularly since a number of branches to be performed is dependent upon which clipping plane is involved. In addition, a size of the clipping program can lead to inefficiencies in terms of enhanced storage requirements.                        
It is against this background that a need arose to develop the apparatus, system, and method described herein.